C. Z. Ning

Lasing in metal-insulator-metal sub-wavelength plasmonic waveguides

We demonstrate lasing in Metal-Insulator-Metal (MIM) waveguides filled with electrically pumped semiconductor cores, with core width dimensions below the diffraction limit. Furthermore these waveguides propagate a transverse magnetic (TM0) or so called gap plasmon mode [1-4]. Hence we show that losses in sub-wavelength MIM waveguides can be overcome to create small plasmon mode lasers at wavelengths near 1500 nm. We also give results showing room temperature lasing in MIM waveguides, with approximately 310 nm wide semiconductor cores which propagate a transverse electric mode.

Optical routing and sensing with nanowire assemblies

The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.

Reflection of guided modes in a semiconductor nanowire laser

We analyze the waveguiding properties of semiconductor (GaN, ZnO, CdS) single nanowire lasers which were recently demonstrated experimentally. In particular, we compute the reflectivity for a few lowest-order guided modes (HE11, TE01, and TM01) from the nanowire facets. The reflectivity is shown to depend strongly on the mode type, lasing frequency and radius of the nanowire. By using the computed reflectivities, we make realisic estimates of the threshold gain and quality factor for the nanowire lasers. Our results shed light on the lasing mechanism of the nanowire lasers.

Continuous Alloy-Composition Spatial Grading and Superbroad Wavelength-Tunable Nanowire Lasers on a Single Chip

By controlling local substrate temperature in a chemical vapor deposition system, we have successfully achieved spatial composition grading covering the complete composition range of ternary alloy CdSSe nanowires on a single substrate of 1.2 cm in length. Spatial photoluminescence scan along the substrate length shows peak wavelength changes continuously from approximately 500 to approximately 700 nm. Furthermore, we show that under strong optical pumping, every spot along the substrate length displays lasing behavior. Thus our nanowire chip provides a spatially continuously tunable laser with a superbroad wavelength tuning range, unmatched by any other available semiconductor-based technology.

Spatial composition grading of quaternary ZnCdSSe alloy nanowires with tunable light emission between 350 and 710 nm on a single substrate

We demonstrated a general methodology of growing spatially composition-controlled alloys by combining spatial source reagent gradient with a temperature gradient. Using this dual gradient method, we achieved for the first time a continuous spatial composition grading of single-crystal quaternary Zn(x)Cd(1-x)S(y)Se(1-y) alloy nanowires over the complete band gap range along the length of a substrate. The band gap grading spans between 3.55 eV (ZnS) and 1.75 eV (CdSe) on a single substrate, with the corresponding light emission over the entire visible spectrum. We also showed that the dual gradient method can be extended to achieve alloy composition control in two spatial dimensions. The unique material platform achieved will open a wide range of applications from color engineered display and lighting, full spectrum solar cells, multispectral detectors, or spectrometer on-a-chip to superbroadly tunable nanolasers. The growth methodology can be extended more generally to other alloy systems.

InxGa1-xas nanowires on silicon: One-dimensional heterogeneous epitaxy, bandgap engineering, and photovoltaics

We report on the one-dimensional (1D) heteroepitaxial growth of In(x)Ga(1-x)As (x = 0.2-1) nanowires (NWs) on silicon (Si) substrates over almost the entire composition range using metalorganic chemical vapor deposition (MOCVD) without catalysts or masks. The epitaxial growth takes place spontaneously producing uniform, nontapered, high aspect ratio NW arrays with a density exceeding 1 × 10(8)/cm(2). NW diameter (∼30-250 nm) is inversely proportional to the lattice mismatch between In(x)Ga(1-x)As and Si (∼4-11%), and can be further tuned by MOCVD growth condition. Remarkably, no dislocations have been found in all composition In(x)Ga(1-x)As NWs, even though massive stacking faults and twin planes are present. Indium rich NWs show more zinc-blende and Ga-rich NWs exhibit dominantly wurtzite polytype, as confirmed by scanning transmission electron microscopy (STEM) and photoluminescence spectra. Solar cells fabricated using an n-type In(0.3)Ga(0.7)As NW array on a p-type Si(111) substrate with a ∼ 2.2% area coverage, operates at an open circuit voltage, V(oc), and a short circuit current density, J(sc), of 0.37 V and 12.9 mA/cm(2), respectively. This work represents the first systematic report on direct 1D heteroepitaxy of ternary In(x)Ga(1-x)As NWs on silicon substrate in a wide composition/bandgap range that can be used for wafer-scale monolithic heterogeneous integration for high performance photovoltaics.

Effective Bloch equations for semiconductor lasers and amplifiers

A set of effective Bloch equations is established for semiconductor bulk or quantum-well media. The model includes the nonlinear carrier-density dependence of the gain and refractive index and their respective dispersions (frequency dependences). A comparative study is performed between the full microscopic semiconductor Bloch equations and this effective model for pulse propagation to show the range of validity of the present model. The results show that this model agrees well with the microscopic model provided carrier depletion is the dominant saturation mechanism relative to the plasma heating. The effective Bloch equations provide an accurate and practical model for modeling amplifiers with pulses of duration greater than a few picoseconds. By capturing the large bandwidth and the carrier density dependence of the gain, it also provides a reliable model for studying the complex spatiotemporal multilongitudinal and transverse mode dynamics of a variety of wide-aperture high-power semiconductor lasers. The model goes beyond the traditional rate equations and is computationally much more efficient to simulate than the full model.

Near-infrared semiconductor subwavelength-wire lasers

We report near-infrared lasing in the telecommunications band in gallium antimonide semiconductor subwavelength wires. Our results open the possibility of the use of semiconductor subwavelength-wire lasers in future photonic integrated circuits for telecommunications applications.

Modal gain in a semiconductor nanowire laser with anisotropic bandstructure

We investigate optical gain for the modes guided by semiconductor nanowires. We focus on optically anisotropic wurtzite-type semiconductors (such as GaN) and the situation when the optical axis of the crystal coincides with the geometrical axis of the nanowire. For GaN nanowire lasers, the calculation of the modal gain requires the knowledge of two confinement factors for a given mode and two gain coefficients for the bulk crystal. We show that the confinement factors for nanowire lasers are very large in comparison to those for heterostructure lasers, and can even exceed unity. To estimate the bulk gain in GaN we use the free-carrier model and emphasize the importance of accounting for anisotropy of gain. Using the calculated confinement factors and bulk gain, we predict that free-standing nanowires with small radius (R /spl lsim/ 70 nm) lase into the HE/sub 11/ mode, thicker nanowires (70 nm /spl lsim/ R /spl lsim/ 90 nm) lase into the TE/sub 01/ mode.

Record performance of electrical injection sub-wavelength metallic-cavity semiconductor lasers at room temperature

We demonstrate a continuous wave (CW) sub-wavelength metallic-cavity semiconductor laser with electrical injection at room temperature (RT). Our metal-cavity laser with a cavity volume of 0.67λ3 (λ = 1591 nm) shows a linewidth of 0.5 nm at RT, which corresponds to a Q-value of 3182 compared to 235 of the cavity Q, the highest Q under lasing condition for RT CW operation of any sub-wavelength metallic-cavity laser. Such record performance provides convincing evidences of the feasibility of RT CW sub-wavelength metallic-cavity lasers, thus opening a wide range of practical possibilities of novel nanophotonic devices based on metal-semiconductor structures.